High resolution radar imaging is a requirement in a variety of remote sensing applications including synthetic aperture radar (SAR) and through-the-wall radar imaging (TWI). Whereas the down-range resolution is mostly controlled by the bandwidth of the transmitted pulse, the cross-range (azimuth) resolution depends on the aperture of the radar array. Generating a large physical aperture is practically achieved by deploying a number of distributed antennas or arrays, each having a relatively small aperture. A distributed setup allows for flexibility of platform placement, reduces the operational and maintenance costs, and adds robustness to sensor failures. Leveraging prior knowledge of the scene, such as sparsity, the precise knowledge of the antenna positions and a full synchronization of received signals has been shown to significantly improve the radar imaging resolution.
Some challenges in radar imaging using distributed sensing is being able to identify locations of antennas due to inaccurate calibration or various position perturbations. Although modern navigation systems such as the Global Positioning System (GPS) can measure positions, the possible position errors due to position perturbations are beyond the scope of high-resolution distributed radar imaging. For example, for a vehicle mounted radar system, as the vehicle is moving along some predesigned trajectory, position perturbations are introduced due to non-smooth road surface or varying driving velocity and direction. These position perturbations can be as large as several wavelengths of the radar center frequency. Consequently, applying standard reconstruction techniques without accounting for the position perturbation produces out-of-focus radar images.
There have been a multitude of solutions that addressed the radar autofocus problem, particularly in the SAR setting, by developing tools that compensate for the antenna position errors. Unfortunately, this problem is ill-posed and solving this problem is a computationally demanding process with difficult to find solution. To that end, some methods impose additional constraints on the autofocusing problem to make this problem tractable. See, e.g., U.S. Pat. No. 8,009,079. Those additional constraints are not always desirable.
Therefore, there is a need for radar imaging systems and methods for autofocusing of distributed antennas having unknown position perturbations.